could fail if the mechanical strength is insufficient. Strength at 1500°C and 1650°C is normally perceived as critical to indicate whether a good ceramic bond forms while the material still retains volume stability. However, too high strength could possibly induce a low thermal shock resistance.
HMoR results after 5 h pre-firing at 1000°C and at 1500°C are shown in Figure 13. The test was conducted at 1500°C after a holding time of 30 min. The HMoR was higher when the samples were pre-fired at 1500°C because sintering took place, forming a stronger ceramic bond. For C5S0 without spinel this effect is less pronounced. When comparing C2S26 andC5S26, HMoR appears to be almost independent of the cement content. Further investigations reported in a previous paper 
show, that higher cement
content such as 10 % appears to reduce HMoR, and for varying Alphabond 300 contents between 2 and 6 % HMoR does not change significantly.
HMoR is an important factor which can affect the performance of a purging plug. Higher HMoR is beneficial for the erosion resistance of purging plug under working conditions, where erosive attack is strong due to the stirring of the steel especially at and around the purging plug. The cement bonded castables show good HMoR values. These are regarded as being adequate for the practical application of the purging plug. The HMoR level of ultra- low cement castable C2S26 and low cement castable C5S26 is around three to four times higher than Alphabond 300 bonded castable. This is attributed to overall stronger sintering reactions in the cement bonded mixes including the formation of new phases such as CA6
shock resistance for the materials pre-fired at the low temperature, and also a higher resistance for the no-cement castable when compared to the cement bonded castables. However, the lower starting strength level of the no-cement material when compared to the cement bonded material also needs to be taken into account, especially at 350°C (8 vs. 18 MPa).
4.3 Wedge splitting and Young’s modulus test results
Fracture curves of castables pre-fired at 350°C for 5 h are shown in Figure 16. The results show that C5S0 and C5S26 have higher maximum vertical forces than A4S26. C5S0 shows slightly higher vertical force than C5S26 which is in line with slightly higher strength values as given in Figures 11 and 12.
Fracture curves of castables pre-fired at 1250°C for 5 h and at 1500°C for 5 h are shown in Figures 17 and 18, respectively. These results show that castables C5S26 and C5S0 exhibit higher vertical forces than the no- cement castable A4S26. The castable C5S26 exhibits the highest vertical force. Compared to the curves of samples pre-fired at a lower temperature (Figure 16), the curves for C5S26 and C5S0 in Figures 17 and 18 are much steeper during the descending load process, which indicates greater brittle cracking in the castables 
. In the case of the no-cement bonded castable . The clearly
higher HMoR of cement bonded castables even at a low cement content of only 2 % (C2S26) is a clear advantage versus the no-cement castables with hydratable alumina when considering the purging plug application, where high erosion resistance is required.
4.2 Thermal shock resistance
The results of thermal shock resistance testing by water quenching are shown in Figures 14 and 15. The relative strength losses after one and three cycles are lower for 400°C pre-firing when compared to 1650°C pre-firing. At 400°C pre-firing, the residual CMoRs are between 30 and 58%, whereas at 1650°C pre-firing, they are between only 10 and 15 %. The residual CMoR in relative terms is higher for the no-cement castable than for the cement bonded castables. This would in general indicate a higher thermal
A4S26 in Figures. 17 and 18, the curves are very similar to that in Figure 16. The slopes of the curves of A4S26 are lower than those of the cement bonded castables, suggesting a more gentle fracture process and less brittle behaviour of the A4S26 sample.
The specific fracture energy of the castables pre-fired at different
temperatures is displayed in Figure 19. These values are calculated from the integral of the load/displacement curves in Figures 16, 17, and 18, and divided by the fracture surface projection area, as previously discussed. The results show, that as temperature increases, the specific fracture energy of C5S26 and C5S0 increases. This increase in the specific fracture energy is a consequence of the increase in the mechanical strength of the castables at higher temperatures, which results in a stronger bonding effect. By contrast, in the case of A4S26, the fracture energy increases up to 1250°C and then decreases at 1500°C. This is attributed to brittle cracking promoted after high temperature treatment as seen in Figures 17 and 18.
Figure 11: Cold modulus of rupture at different pre- firing temperatures
Figure 12: Cold crushing strength at different pre- firing temperatures
Figure 13: HMoR after pre-firing at 1000°C / 5 h and 1500°C / 5 h respectively (tested at 1500°C / 0.5 h)
Figure 14: Residue strength % (CMoR) after 1 and 3 water quenches (pre-fired at 400°C / 5 h)
July 2018 Issue
Figure 15: Residue strength % (CMoR) after 1 and 3 water quenches (pre-fired at 1650°C / 5 h)
ENGINEER THE REFRACTORIES
Figure 16: Load/displacement curves of the castables pre-fired at 350°C for 5 h
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